1. Technical Field
This disclosure relates to a single mode high power fiber laser system configured with a multimode fiber capable of guiding light in a substantially fundamental mode.
2. Known Art Discussion
Numerous applications of fiber laser systems are in need of a high-power, high-quality beam. Fiber lasers utilizing SM active fibers are limited in power due to the onset of optical nonlinearities. One common solution is the use a MM active fiber capable of supporting a few high-order modes (HOM) but configured to prevent the excitation and amplification of these HOMs.
Yet the power scaling of single mode high power (SMHP) fiber laser systems with such MM fibers is also somewhat limited by the presence of nonlinearities including, but not limited to, Stimulated Raman Scattering (SRS). Perhaps one the most efficient practical approaches, leading to a relatively high optical nonlinearity threshold, is to decrease the power density inside the core of a MM fiber by increasing the core diameter, decreasing a numerical aperture and also decreasing the effective length of non-linear interaction. Unfortunately, this geometry is not easily attainable for the following reasons. First, the increase of the core diameter results in the increased number of HOMs which can be easily excited that detrimentally affects the quality of the output beam. Second, the manufacturing of high quality fibers with truly very low Δn is highly challenging. Third, such fibers are sensitive to bending loads.
A need, therefore, exists for a SMHP system provided with at least one active MM fiber substantially free from problems associated with the known prior art systems.
Another need exists for a SMHP system provided with a MM active fiber which has a refractive core index with the increased effective area of the excited fundamental mode and higher thresholds for nonlinearities if compared to those of the known SMHP systems.
Still another need exists for a SMHP system with an active multimode MM fiber which is configured with a dopant profile capable of amplifying substantially only a fundamental mode while minimizing the possibility of coupling thereof to peripheral and central symmetrical high order modes.